Technical Field
Embodiments of the present disclosure relate to a malfunction diagnosis apparatus and a malfunction diagnosis method.
Description of Related Art
A NOx selective catalytic reduction catalyst (hereinafter, simply referred to as NOx catalyst) may purify NOx contained in exhaust gas from an internal combustion engine with the use of ammonia as a reducing agent. For example, an addition valve that adds ammonia or ammonia precursor into exhaust gas may be installed upstream of the NOx catalyst in the exhaust gas flow direction. An example of the ammonia precursor may be urea. Ammonia precursor or ammonia may also be collectively referred to as reducing agent.
Published Japanese Translation of PCT Application No. 2009-510324 (JP 2009-510324A) describes an exhaust aftertreatment system that may include a NOx sensor provided downstream of a NOx catalyst. The exhaust aftertreatment system described in JP 2009-510324A may determine that there is a malfunction in supply of reducing agent if a detected value of the NOx sensor remains unchanged or decreases at the time when an addition valve is operated such that the amount of reducing agent that is supplied to the NOx catalyst is increased. In the case where the amount of reducing agent supplied from the addition valve per unit time is reducing, the amount of supplied reducing agent may approach an appropriate value when the addition valve is operated such that the amount of supplied reducing agent increases. Therefore, a NOx purification rate decreases, with the result that the detected value of the NOx sensor decreases. When reducing agent is not supplied from the addition valve, reducing agent may not be supplied even when the addition valve is operated such that the amount of supplied reducing agent increases, so the detected value of the NOx sensor remains unchanged.